Abstract
Even though NK cells have been named for their spontaneous cytotoxicity, a large subpopulation of human NK cells primarily responds to activation with cytokine secretion and not killing. These CD56brightCD16(-) NK cells are abundant in secondary lymphoid tissues, can restrict pathogens that have breached mucosal barriers, and assist Th1 polarization during immune response priming. This exciting new aspect of NK-cell biology is discussed in this viewpoint.